We have developed a fluorescent antibody technique for rapid identification of group B streptococci in human specimens. The technique has a sensitivity of 52% for detection of maternal vaginal colonization within 6 hours of culture; 93% after overnight (greater than 18 hours) incubation. The FA technique is being used as a semiquantitative method of assessing the risk of vertical transmission of group B streptococci from parturient women to their newborn infants. Using biologic characteristics of group B streptococci which permit specific identification of strains (serotype, antibiogram, neuramindase production, chick embryo virulence, and bacteriophage type we are studying nosocomial transmission in a high-risk nursery environment. Using the risk factors of premature labor and prolonged rupture of maternal membranes as criteria for selection, we are conducting a controlled study of ampicillin prophylaxis of neonatal group B streptococcal infections in colonized women in labor.